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November 1, 2002|Volume 31, Number 9



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Ramamurti Shankar



Physicist offers 'Yogi Berra' guide to quantum world

Ramamurti Shankar, chair of the physics department, faced a daunting challenge when he was invited by Graduate School Dean Susan Hockfield to explain quantum theory to non-physicists without using mathematical equations -- and he almost pulled it off.

One equation did sneak into his talk, titled "When you come to a fork in the road, take it -- Yogi Berra's introduction to the quantum world," but the rest of the lecture was free of math and full of humor.

"When I first heard this quotation from the great Yogi -- 'When you come to a fork in the road, take it' -- I thought it was funny," Shankar said. "But one day I was thinking, and I realized this is not funny at all. This is exactly what electrons do. All the particles on the quantum level, when they come to the fork, take it. So I said, as a physicist, I should not be laughing at this particular joke anymore."

Speaking to an audience of about 400 students and faculty members at the opening event of this year's "In the Company of Scholars" lecture series, Shankar used the sayings of the legendary Yankees catcher/manager to help explain the nature of quantum mechanics. In preparing for the talk, Shankar went to the website that has all the sayings of Yogi Berra and found that "Each one of them says something very profound and deep about physics," he said.

Quantum mechanics is the study of the structure and behavior of subatomic particles, atoms and molecules. Its implications affect everything from the smallest muons and gluons to entire galaxies. It is a sophisticated and counter-intuitive branch of physics, said Shankar, noting that trying to explain it without what he called "heavy-duty mathematics" was like trying to explain "Romeo and Juliet" to someone who doesn't understand the concept of love.

In several areas of physics, Shankar said, there is "something you thought you could do, that you cannot do." For example, you cannot cool water down to an infinitely low temperature, because once you reach absolute zero, you can't go any lower. "Now, that's a surprise," commented the scientist, "because you might think that you could build more and more powerful refrigerators and keep on cooling things, but you cannot. ...

"Similarly, relativity has something to say about light," he continued. "Alfred E. Newman, who appeared in Mad Magazine, used to say, 'This is what I know about the velocity of light: It gets to you too early in the morning.'"

Citing Albert Einstein, Shankar said, "'Nothing can travel faster than the speed of light.' That sentence means the same to you as to any professional physicist." When it comes to extreme circumstances like absolute zero and the speed of light, common sense logic doesn't agree with physics, and in that contest, common sense always loses, asserted the scholar.

"In quantum mechanics, there are so many things you cannot do, it's like a religion," he joked. "Quantum mechanics allow tiny particles to do weird things."

Among the unexpected findings, he said, "It is wrong ,to assume there's any trajectory to an electron." Just because an electron is sighted first at one location and later at another, he noted, doesn't mean it has traveled along a continuous path, the way a football does when it's thrown. Again quoting Yogi Berra's words, "You can see a lot by just observing," Shankar pointed out that, in the case of electrons, if they are not observed, they have no trajectory. Anyone who insists on the common sense notion that everything that moves has a trajectory will be proven wrong, he added.

In fact, he emphasized, particles like electrons do not have attributes like position or velocity or trajectories until they are measured. Before it is given a definite location through a position measurement, Shankar said, the electron is in "a state of limbo that has no analog in our macroscopic world."

To demonstrate the complexities of studying electrons, Shankar proposed to the audience a "gedanken" or thought experiment invented by Richard P. Feynman, called the Double Slit Experiment.

"Einstein was a master of these experiments. All of us are becoming masters, because we've got no money for real experiments. Some of us don't even have money to do gedanken experiments," he quipped.

Shankar explained how the Double Slit Experiment works: You imagine a room divided down the middle by a partition. There is an electron source at one end of the room and, beyond the partition, an electron detector at the other end. The partition has two slits that can be opened and closed. When one slit is open, and one electron is emitted from the source at the same speed every second, a predictable number of electrons will arrive at the detector at the other end of the room (say, five electrons a minute).

However, explained Shankar, when both slits are open at the same time and electrons are emitted from the source at the same frequency and velocity as before, there are some locations behind the screen where, instead of twice the number of electrons arriving at the detector (10 per minute), no electrons at all are detected -- contrary to common sense and classical Newtonian mechanics. Likewise, he noted, there are other locations where you get more electrons than you expect (such as 20 per minute).

Shankar posited a number of explanations of why no electrons are detected when both slits are open, including the "Oliver Stone conspiracy theory," in which the electrons have "devious little minds and say, 'Hey, when they open both, nobody go there! Let's screw these guys up.'" But electrons being inanimate objects, he conceded, that's not especially likely.

Dismissing the conspiracy theory and other playful explanations, Shankar finally resorted to using his sole equation -- one related to wave mechanics using Planck's constant. He demonstrated, theoretically, that when both slits are open, the electrons' arrival rate can be calculated by taking contributions from both slits as in wave propagation -- so that in some locations, the waves could cancel each other while in other places they reinforced each other. He pointed out that, even with the waves, only the odds of what would happen could be calculated; the deterministic world of Newton was gone.

Thus, he said, electrons are like particles in that in any one trail, they trigger only one detector on the back of the slits. But, he added, if you want to know the likelihood of whether they will land here or there, you need to introduce a wave or resort to Feynman's procedure of summing contributions over possible paths.

Thus, as Shankar noted earlier, when electrons come to a fork in the road, they take both paths.

-- By Gila Reinstein


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Physicist offers 'Yogi Berra' guide to quantum world

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Campus Notes


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